PURPOSES: To identify factors associated with physical inactivity among adolescents in Ho Chi Minh City (HCMC). METHODS:: A cross-sectional survey was conducted in 2004 among 2684 junior high school students using multistage cluster sampling. Time spent for physical and sedentary activities was collected with a validated questionnaire. Anthropometry, parental/family characteristics, neighborhood, and school environments were also measured. Level of activity was classified as physically active or inactive using standard criteria. Potential risk factors for physical inactivity were identified with hierarchical multivariate logistic regression, stratified by gender. RESULTS:: Physical inactivity was positively associated with child's overweight status (odds ratio [OR] = 2.5, 95% confidence interval [CI] = 1.9, 3.2), passive transportation (OR = 4.2, 95% CI = 3.3-5.2), not having recess exercises (OR = 1.3, 95% CI = 1.1-5.6), availability of a nearby game shop (OR = 1.8, 95% CI = 1.5-2.2), more time spent playing video games, watching television, and studying after class (OR = 2.3, 95% CI = 1.7-3.1; OR = 1.5, 95% CI = 1.2-1.9; and OR = 1.8, 95% CI = 1.3-2.4, respectively). The following factors decreased the odds of inactivity: schools having one to two sport meetings per year (OR = 0.7, 95% CI = 0.5-1.0), availability of a play yard (OR = 0.7, 95% CI = 0.6-0.9), older age group (OR = 0.2, 95% CI = 0.1-0.2), and male gender (OR = 0.6, 95% CI = 0.5-0.7). In boys, transportation, school sport meeting, availability of game shop near by, family's economic status, parental body mass index status, and time spent watching television were strongly associated with physical inactivity. In girls, time spent for study after class was one of the most important factors related to students' physical inactivity. CONCLUSIONS:: Physical activity among adolescents in HCMC is related to household, school, and community factors. These factors and gender differences should be taken into account when promoting physical activity in adolescents.